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<div class="titlepage"><div><div><h4 class="title">
<a name="beast.ref.boost__beast__http__async_read_header"></a><a class="link" href="boost__beast__http__async_read_header.html" title="http::async_read_header">http::async_read_header</a>
</h4></div></div></div>
<p>
        <a class="indexterm" name="idp104117760"></a>
Read a header from a stream asynchronously using a
        parser.
      </p>
<h5>
<a name="beast.ref.boost__beast__http__async_read_header.h0"></a>
        <span class="phrase"><a name="beast.ref.boost__beast__http__async_read_header.synopsis"></a></span><a class="link" href="boost__beast__http__async_read_header.html#beast.ref.boost__beast__http__async_read_header.synopsis">Synopsis</a>
      </h5>
<p>
        Defined in header <code class="literal">&lt;<a href="../../../../../../boost/beast/http/read.hpp" target="_top">boost/beast/http/read.hpp</a>&gt;</code>
      </p>
<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span>
    <span class="keyword">class</span> <a href="../../../../../../doc/html/boost_asio/reference/AsyncReadStream.html" target="_top"><span class="bold"><strong>AsyncReadStream</strong></span></a><span class="special">,</span>
    <span class="keyword">class</span> <a class="link" href="../concepts/DynamicBuffer.html" title="DynamicBuffer"><span class="bold"><strong>DynamicBuffer</strong></span></a><span class="special">,</span>
    <span class="keyword">bool</span> <span class="identifier">isRequest</span><span class="special">,</span>
    <span class="keyword">class</span> <span class="identifier">Derived</span><span class="special">,</span>
    <span class="keyword">class</span> <a href="../../../../../../doc/html/boost_asio/reference/ReadHandler.html" target="_top"><span class="bold"><strong>ReadHandler</strong></span></a><span class="special">&gt;</span>
<a href="../../../../../../doc/html/boost_asio/reference/asynchronous_operations/automatic_deduction_of_initiating_function_return_type.html" target="_top"><span class="emphasis"><em>DEDUCED</em></span></a>
<span class="identifier">async_read_header</span><span class="special">(</span>
    <span class="identifier">AsyncReadStream</span><span class="special">&amp;</span> <span class="identifier">stream</span><span class="special">,</span>
    <span class="identifier">DynamicBuffer</span><span class="special">&amp;</span> <span class="identifier">buffer</span><span class="special">,</span>
    <span class="identifier">basic_parser</span><span class="special">&lt;</span> <span class="identifier">isRequest</span><span class="special">,</span> <span class="identifier">Derived</span> <span class="special">&gt;&amp;</span> <span class="identifier">parser</span><span class="special">,</span>
    <span class="identifier">ReadHandler</span><span class="special">&amp;&amp;</span> <span class="identifier">handler</span><span class="special">);</span>
</pre>
<h5>
<a name="beast.ref.boost__beast__http__async_read_header.h1"></a>
        <span class="phrase"><a name="beast.ref.boost__beast__http__async_read_header.description"></a></span><a class="link" href="boost__beast__http__async_read_header.html#beast.ref.boost__beast__http__async_read_header.description">Description</a>
      </h5>
<p>
        This function is used to asynchronously read a header from a stream into
        a subclass of <a class="link" href="boost__beast__http__basic_parser.html" title="http::basic_parser"><code class="computeroutput"><span class="identifier">http</span><span class="special">::</span><span class="identifier">basic_parser</span></code></a>. The function call
        always returns immediately. The asynchronous operation will continue until
        one of the following conditions is true:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
            <a class="link" href="boost__beast__http__basic_parser/is_header_done.html" title="http::basic_parser::is_header_done"><code class="computeroutput"><span class="identifier">http</span><span class="special">::</span><span class="identifier">basic_parser</span><span class="special">::</span><span class="identifier">is_header_done</span></code></a> returns <code class="computeroutput"><span class="keyword">true</span></code>
          </li>
<li class="listitem">
            An error occurs.
          </li>
</ul></div>
<p>
        This operation is implemented in terms of one or more calls to the stream's
        <code class="computeroutput"><span class="identifier">async_read_some</span></code> function,
        and is known as a <span class="emphasis"><em>composed operation</em></span>. The program must
        ensure that the stream performs no other reads until this operation completes.
        The implementation may read additional octets that lie past the end of the
        message being read. This additional data is stored in the dynamic buffer,
        which must be retained for subsequent reads.
      </p>
<p>
        If the stream returns the error <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">error</span><span class="special">::</span><span class="identifier">eof</span></code> indicating
        the end of file during a read, the error returned from this function will
        be:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
            <a class="link" href="boost__beast__http__error.html" title="http::error"><code class="computeroutput"><span class="identifier">http</span><span class="special">::</span><span class="identifier">end_of_stream</span></code></a>
            if no octets were parsed, or
          </li>
<li class="listitem">
            <a class="link" href="boost__beast__http__error.html" title="http::error"><code class="computeroutput"><span class="identifier">http</span><span class="special">::</span><span class="identifier">partial_message</span></code></a>
            if any octets were parsed but the message was incomplete, otherwise:
          </li>
<li class="listitem">
            A successful result. A subsequent attempt to read will return <a class="link" href="boost__beast__http__error.html" title="http::error"><code class="computeroutput"><span class="identifier">http</span><span class="special">::</span><span class="identifier">end_of_stream</span></code></a>
          </li>
</ul></div>
<h5>
<a name="beast.ref.boost__beast__http__async_read_header.h2"></a>
        <span class="phrase"><a name="beast.ref.boost__beast__http__async_read_header.parameters"></a></span><a class="link" href="boost__beast__http__async_read_header.html#beast.ref.boost__beast__http__async_read_header.parameters">Parameters</a>
      </h5>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                <p>
                  Name
                </p>
              </th>
<th>
                <p>
                  Description
                </p>
              </th>
</tr></thead>
<tbody>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="identifier">stream</span></code>
                </p>
              </td>
<td>
                <p>
                  The stream from which the data is to be read. The type must support
                  the AsyncReadStream concept.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="identifier">buffer</span></code>
                </p>
              </td>
<td>
                <p>
                  A DynamicBuffer holding additional bytes read by the implementation
                  from the stream. This is both an input and an output parameter;
                  on entry, any data in the dynamic buffer's input sequence will
                  be given to the parser first.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="identifier">parser</span></code>
                </p>
              </td>
<td>
                <p>
                  The parser to use. The object must remain valid at least until
                  the handler is called; ownership is not transferred.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="identifier">handler</span></code>
                </p>
              </td>
<td>
                <p>
                  Invoked when the operation completes. The handler may be moved
                  or copied as needed. The equivalent function signature of the handler
                  must be:
                </p>
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting"> <span class="keyword">void</span> <span class="identifier">handler</span><span class="special">(</span>
    <span class="identifier">error_code</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">error</span><span class="special">,</span>        <span class="comment">// result of operation,</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">bytes_transferred</span>   <span class="comment">// the number of bytes transferred to the parser</span>
<span class="special">);</span>
</pre>
                <p>
                  Regardless of whether the asynchronous operation completes immediately
                  or not, the handler will not be invoked from within this function.
                  Invocation of the handler will be performed in a manner equivalent
                  to using <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">io_context</span><span class="special">::</span><span class="identifier">post</span></code>.
                </p>
              </td>
</tr>
</tbody>
</table></div>
<h5>
<a name="beast.ref.boost__beast__http__async_read_header.h3"></a>
        <span class="phrase"><a name="beast.ref.boost__beast__http__async_read_header.remarks"></a></span><a class="link" href="boost__beast__http__async_read_header.html#beast.ref.boost__beast__http__async_read_header.remarks">Remarks</a>
      </h5>
<p>
        The implementation will call <a class="link" href="boost__beast__http__basic_parser/eager.html" title="http::basic_parser::eager"><code class="computeroutput"><span class="identifier">http</span><span class="special">::</span><span class="identifier">basic_parser</span><span class="special">::</span><span class="identifier">eager</span></code></a> with the value <code class="computeroutput"><span class="keyword">false</span></code> on the parser passed in.
      </p>
<p>
        Convenience header <code class="literal">&lt;<a href="../../../../../../boost/beast/http.hpp" target="_top">boost/beast/http.hpp</a>&gt;</code>
      </p>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright &#169; 2016, 2017 Vinnie Falco<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
</div></td>
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